Andrzej W. Pacek

On the Sauter mean diameter and size distributions in turbulent liquid/liquid dispersions in a stirred vessel

Abstract It has been customary to relate the Sauter mean drop size, d 32 , to Weber number, We , by the relationship d 32 D∝We -0.6 . This relationship comes from the assumptions that d 32 is a constant fraction of d max , where d max is the maximum stable drop size and that d max can be predicted from theoretical considerations if Kolmogoroffs theory of isotropic turbulence is used for estimating the disruptive forces. Here it is shown, firstly on theoretical grounds, that the assumption d 32 ∝ d max is not justified; and secondly that experimental results from 18 different runs neither support d 32 = Ad max where A is system and agitation conditions independent nor that the exponent on We is −0.6. Further, though cumulative volume size distributions indicate self-similarity, as suggested previously, the number probability density distributions show strong bi-modality at low speed and low dispersed-phase concentration which lessens with increasing concentration and becomes uni-modal at high speeds. This study suggests that in spite of the great deal of work which has already been done, more is required in which the relationship between mean drop size and drop size distributions and agitation conditions over a wider range of concentrations are further investigated.

Bubble sizes in electrolyte and alcohol solutions in a turbulent stirred vessel

Bubble size distributions have been measured by a new video technique at 3 points near the wall in a vessel of 150 mm diameter air-sparged at ∼1 vvm agitated by a Rushton turbine at an energy dissipation rate of ∼1Wkg-1. Water and solutions of electrolytes and alcohols were used. These solutes give surface tensions less than water (alcohols) and greater than water (electrolytes) and concentrations were chosen to produce solutions which, based on work in bubble columns and coalescence cells, can be considered partially-coalescing and non-coalescing. Regardless of surface tension, the bubble sizes in the non-coalescing solutions were approximately the same and much less than water, whilst those in the partiallycoalescing case where the surface tension was approximately equal to that of water, gave intermediate sizes. Thus, the Weber number cannot correlate such results. On the other hand, the concept of bubble sizes being controlled by coalescence-inhibitionafter initial break-up works well. In all cases, bubbles as small as 40 μm were found and even in water some 40% were below 300 μm, the smallest size practicably measurable by a capillary technique. Surprisingly, the bubble size decreased with vessel height and possible reasons for this are discussed.

The influence of impeller type on mean drop size and drop size distribution in an agitated vessel

Abstract In most work on agitated liquid–liquid dispersions, Rushton turbines have been used. Here, mean drop size and drop size distributions are reported for six different impellers covering 3 generic types over a range of mean specific energy dissipation rates. Both viscous and non-viscous dispersed phases have been used at concentrations by volume of 1 and 5%. It has been found that at the same mean specific energy dissipation rate, low power number impellers (whether of the so-called “ultra-high shear” or “high flow” type) all produced similar sized drops at equilibrium which were much smaller than those found with two “high shear”, high power number impellers, i.e., the standard Rushton turbine and another six-blade disc impeller. By considering the energy dissipation rate to be confined to the impeller swept volume, these equilibrium drop size could be approximately correlated. The low power number impellers also achieved that equilibrium more rapidly and the drop size distributions in the dispersions produced by them were narrower than those formed when agitated by the Rushton turbine and the other six-blade disc impeller. However, further analysis of the flow in the impeller region and the inclusion of advanced coalescence models would appear to be required in order to enhance the interpretation of these results.

Fluidisation of fine and very dense hardmetal powders

Abstract The fluidisation characteristics of a hardmetal powder, of extremely high density and small primary particle size such that it belongs to Group C in Geldarts classification, has been investigated. Surprisingly, it was found that in this case the powder could be fluidised via self-agglomeration, though such behaviour is not predicted from the primary particle diameter and/or its density, or the Geldart classification. When the gas is flowing for the first time through a bed of this powder, three regimes of two phase flow patterns were distinguished with increasing gas velocity: gas—‘solid powder’; gas—‘solid powder’ and agglomerates; gas—agglomerates. Subsequently, only the third condition is found with the agglomerate size dependent on gas velocity. A theoretical analysis of the first pattern is also presented, as well as a general qualitative description of the whole process.

Effects of agitation and scale-up on drop size in turbulent dispersions: allowance for intermittency

Abstract Experimental and theoretical work has recently shown that classical drop size correlations have significant limitations. In particular, that work indicated a slow drift towards smaller drops when agitation is maintained, as well as smaller drops and faster break-up when scaling up at constant power per unit volume. Moreover, the exponent on Weber number fell below −0.6. It was considered that the phenomenon of turbulent intermittency was the mechanism causing the limitations. Here, these ideas are explored farther using equations for stable drop size and drop break-up in intermittent turbulence, the latter being modelled by a multifractal spectrum. These equations are then successfully applied to new drop size measurements for two geometrically similar stirred tanks having different scales, giving further support for the need to consider the phenomenon of intermittency when modelling mixing processes in stirred tanks in the turbulent regime.

On the Structure of Turbulent Liquid-Liquid Dispersed Flows in an Agitated Vessel

Abstract The structure and stability of dispersed liquid—liquid flow have been studied in a stirred vessel under semi-batch (wash-out) and batch conditions. Four aqueous-organic systems were used with the density and viscosity of the organic phase close to and straddling the values for water. A recently developed video technique was employed, capable of giving sharp images of the dispersed phase droplets down to 25 μm. In the wash-out system, the dispersed phase fraction was increased from zero continuously until phase inversion occurred; and in the batch case, concentration from 0.05 in steps of 0.1 volume phase fraction up to phase inversion were used. In all cases, for water-in-oil dispersions above ∼ 0.25 volume fraction up to inversion, droplets of oil were found in the drops of water, and within a few seconds of phase inversion, such a structure disappeared. Conversely, for all fractions of oil-in-water, droplets-in-drops were not observed; but, in less than a second, following inversion, droplets of oil in drops of water formed. These findings are used to explain qualitatively: (i) the larger drop sizes found in water-in-oil dispersions at the same concentrations compared to those in oil-in-water ones under equivalent hydrodynamic conditions; (ii) the lower concentration in water-in-oil systems at which phase inversion occurs compared to oil-in-water systems; (iii) the presence of a delay time with water-in-oil inverting to oil-in-water compared to the absence of a delay time in the reverse case. It would appear that the general fluid dynamic treatment of such systems which only utilises the two-phase densities and viscosities and the symmetrical property, interfacial tension, is unable to account for these observations.

Study of drop and bubble sizes in a simulated mycelial fermentation broth of up to four phases.

The mean sizes and size distributions of air bubbles and viscous castor oil drops were studied in a salt-rich aqueous solution (medium), first separately, and then simultaneously as a three-phase system. The dispersion was created in a 150-mm-diameter stirred tank equipped with a Rushton turbine, and the sizes were measured using an advanced video technique. Trichoderma harzianum biomass was added in some experiments to study the effect of a solid phase under unaerated and aerated conditions to give either three-or four-phase systems. In all cases, the different dispersed phases could be clearly seen. Such photoimages have never been obtained previously. For the three phases, air-oil-medium, aeration caused a drastic increase in Sauter mean drop diameter, which was greater than could be accounted for by the reduction in energy dissipation on aeration. Also, as in the unaerated case, larger drops were observed as the oil content increased. On the other hand, mean bubble sizes were significantly reduced with increasing oil phase up to 15% with bubbles inside many of the viscous drops. With the introduction of fungal biomass of increasing concentration (0.5 to 5 g L(-1)) under unaerated conditions, the Sauter mean drop diameter decreased. Finally, in the four-phase system (oil [10%]-medium-air-biomass) as found in many fermentations, all the phases (plus bubbles in drops) could clearly be seen and, as the biomass increased, a decrease in both the bubble and the drop mean diameters was found. The reduction in size of bubbles (and therefore increase in interfacial area) as the oil and bio- mass concentration increased provides a possible explanation as to why the addition of an oil phase has been reported to enhance oxygen transfer during many fermentations.

Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nanopowder

The kinetics of deagglomeration in diluted suspensions of goethite nanopowder, as well as the rheology and morphology of the resulting suspensions, strongly depends on pH. At pH 3, nanopowder can be dispersed as separate nanoparticles, and the resulting suspension is Newtonian, with the viscosity only marginally higher than the viscosity of water. At pH between 5 and 12, nanoparticles tend to reaggregate and form weak aggregates/flocs. Morphology changes from a Newtonian suspension of primary nanoparticles to a non-Newtonian, shear-thinning suspension of large, porous, interconnected flocs with the yield stress reaching a maximum at an isoelectric point. The effect of pH on morphology and rheology is reversible, and as pH is reduced to 3, the suspension becomes Newtonian, with viscosity marginally higher than the viscosity of water. The rheological models based on DLVO theory do not allow prediction of the effect of pH on viscosity and yield stress, but the flow curves of goethite suspensions can be described by a fractal model with five adjustable parameters.

The effect of volume fraction and impeller speed on the structure and drop size in aqueous/aqueous dispersions

Abstract The mean drop size and the structure of two-phase aqueous/aqueous dispersions, one-phase sodium alginate-rich of viscosity ∼0.25 Pa s and the other sodium caseinate-rich of viscosity ∼0.022 Pa s , have been measured in an unbaffled vessel fitted with a helical screw impeller. The measurements were carried out over a range of volume fractions and at Reynolds numbers in the range from laminar to low transitional. In addition, the interfacial tension between the two phases has been measured in situ using a recently developed drop retraction technique, which, for the first time, has been successfully applied at a high volume fraction of the dispersed phase. At low volume fractions of the viscous phase (viscosity ratio, λ = μ d / μ c ≈10), drops of that phase are seen much as in equivalent aqueous/oil dispersions but the functionality between the drop size and impeller speed is different. As the volume fraction of the viscous phase increases, the structure first changes to a striated one, something never seen in “pure” oil/aqueous dispersions. The striated structure also evolves into complex (droplets-in-drops) in samples withdrawn from the vessel and within the vessel when stirring is stopped. This implies that the system is in a phase inversion region, but contrary to oil/water dispersions, there is not a rapid switch from one phase being continuous to the other, i.e. the phase inversion region appears to be very stable in time. On a further increase of the volume fraction of the viscous phase, phase inversion occurs when stirring but a striated structure continues to exist, i.e. there is no dramatic change of structure as found with aqueous/oil dispersions undergoing phase inversion. However, when a sample is withdrawn or the impeller is stopped, the complex droplets-in-drops formation no longer appears and only a simple dispersed structure develops. Only at very low speeds and volume fractions of the low viscosity dispersed phase, i.e., λ ∼0.1, do drops re-appear in the vessel when stirring. Overall, it can be concluded that there is a very significant difference in the behavior of oil/aqueous and aqueous/aqueous dispersions.

Fabrication and Characterisation of a Novel Pellicular Adsorbent Customised for the Effective Fluidised Bed Adsorption of Protein Products

A dense pellicular solid matrix has been fabricated by coating 4% agarose gel on to dense zirconia-silica (ZS) spheres by water-in-oil emulsification. The agarose evenly laminated the ZS bead to a depth of 30 μm, and the resulting pellicular assembly was characterised by densities up to 2.39 g/mL and a mean particle diameter of 136 μm. In comparative fluidisation tests, the pellicular solid phase exhibited a two-fold greater flow velocity than commercial benchmark adsorbents necessary to achieve common values of bed expansion. Furthermore, the pellicular particles were characterised by improved qualities of chromatographic behaviour, particularly with respect to a three-fold increase in the apparent effective diffusivity of lysozyme within a pellicular assembly modified with Cibacron Blue 3GA. The properties of rapid protein adsorption/desorption were attributed to the physical design and pellicular deployment of the reactive surfaces in the solid phase. When combined with enhanced feedstock throughput, such practical advantages recommend the pellicular assembly as a base matrix for the selective recovery of protein products from complex, particulate feedstocks (whole fermentation broths, cell disruptates and biological extracts).